Support :placeholder-shown pseudo-class

[Zirro]

The :placeholder-shown pseudo-class has pretty good support in recent browsers. It matches <input> elements which are currently showing their placeholder text. This can probably be implemented by checking that the element has a placeholder, but isn't currently focused.

Specification: https://drafts.csswg.org/selectors-4/#placeholder

Test: https://github.com/w3c/web-platform-tests/blob/master/html/semantics/selectors/pseudo-classes/placeholder-shown-type-change.html

[dperini]

This one was already on the list of candidates, I just needed a push from interested people. It does not look difficult to implement. I am going to add this too.

[dperini]

@Zirro ok, started working on it.

Added initial handling code for ":placeholder-shown" in 3758218

Have not tested yet but during the next weekend I might have more time for this.

[Zirro]

This has been implemented now 🎉